1. Field of the Invention
The present invention refers to an electrical cable for performing stimulations and/or measurements inside a human or animal body to be used in conjunction with electrical or electronic instruments, such as a cardiac pacemaker. The cable may comprise one or more bundles of electrically conducting wires. It should be understood, therefore, that whenever the term bundles is referred to in plural, the singular is meant to be included, unless the formation under discussion clearly includes several bundles, as for example a plaited, woven or twisted formation. The invention equally refers to a method for manufacturing the cable of the invention.
2. Description of the Prior Art
Cables of this type are commonly used in medicine and surgery in conjunction with electrical and/or electronic equipment, and are implanted together with such equipment into a human or animal body for the purpose of stimulation, measurement and recording of physiological variables, and thus for research,therapy and the management of patients with cardiological, neurological and other diseases. The cardiac pacemaker is a classical example of such equipment.
In an article titled "Requirements of the Ideal Pacemaker Lead" by A. Sinnaeve, R. Willems and R. Stroobandt, appeared in the publication "Pacemaker Leads" (pages 47-55), edited by A.E. Aubert and H. Ector (Elsevier Science Publications B.V., Amsterdam, 1985), the authors elaborate in detail on the requirements of pacemaker leads. Such leads (referred to as cables in the present specification) are subject to repetitive mechanical stresses, by being moved and deformed in the course of every heartbeat, adding up to approximately 37 million of repetitions each year. To these are added the stresses due to the movements of breathing, the movements of the arms, and the other bodily movements. There result in the lead bending stresses and deformations, primarily, but also torsional stresses and deformations, as well as tension stresses and elongations. The leads or cables are required to stand up to these mechanical loads without breaking, they should have a preferably low electrical resistance. They should also have high corrosion resistance in regard to blood and other bodily fluids, and be as thin as possible.
Many of the known pacemaker leads comprise wires of circular or rectangular cross-section. The wires frequently consist of a platinum-iridium alloy, of alloys containing silver or gold, of the alloys commercially available under the names Elgiloy and MP35N and comprising cobalt, chromium, nickel and other components, or of tungsten, stainless steel and aluminium or copper. The cables are commonly provided with insulation on the outside and sometimes they comprise a core made of insulating material. In this connection reference is made to the article by Sinnaeve et al. cited above, as well as to US-PS 4,640,983. In most of the known cables, having wires of circular cross-section, the diameters of these wires are in the order of magnitude of about 0.1 mm or more. In US-PS 4,640,983 are recommended diameters of 20 to 80 micrometers for wires of circular cross-section, whereas the wires actually used are specified as 50 micrometers in diameter.
In most of the known cables the wires are wound individually. In the cited US-PS 4,640,983 the wires are wound in bundles, to helices, the helical pitch of the wires, or the wire bundles, being generally made rather small, resulting in adjacent helical windings abutting against each other, or pretty nearly so. To be sure, the wire or fiber pliability (flexibility) may be improved in this way, as compared to wires disposed in straight line. However, when the wires are bent to small radii of curvatures, fatigue fractures may occur, this being also a function of the type of wire used. If, due to such a fracture, current interruption takes place in the cable of a cardiac pacemaker, the patient may be instantly killed. Such fractures are therefore dangerous possibilities.
In US-PS 4,640,983, the cable is wound into helices consisting of bundles,each comprising seven twisted wires. These cables are said to show fatigue resistance down to values of bending radii as low as 1.5 mm. However, the task of shaping bundles of intertwisted wires into coils to follow the form of multiple threads is rather difficult and elaborate.
In all known cables having wires wound to coils and their winding substantially abutting against one another, the wound-off length of the wires is made to be a multiple of the length of the corresponding cable. This results in the disadvantage, that the electrical resistance is largely increased. To compensate for this disadvantage the wires, or at least some of them may be made of a material having a low specific electrical resistance, such as platinum-iridium, silver, or copper. To be sure these materials have unfavourable mechanical properties. Platinium and iridium as well as silver are relatively expensive, whereas copper may become toxic, when brought in contact with bodily fluids or cells.
If a cable of wires and plastic insulation surrounding the wires is implanted into a human or animal body the possibility may arise that the insulation will suffer damage when during cable insertion,or later on during any movements of the bodily parts that accommodate the cable. The result may be that the electrical energy supplied will get short-circuited into the human or animal body before reaching the end of the cable. This could happen with all of the wire materials specified before except perhaps with aluminium. For the sake of clarification let some additional geometrical and physical variables and their mutual relationships be now explained. In the following, the term critical radius of curvature r.sub.c will be used to refer to the smallest radius of curvature to which a cable wire or fiber may be bent, without causing any fracture. If a wire or a fiber of a cross-sectional moment of inertia I, and a section modulus W, and of a material having a modulus of elasticity E and a maximum allowable tensile stress .sigma..sub.z, then the critical radius of curvature may be expressed by the formula: ##EQU1##
If the wire or fiber has a circular cross-section and a diameter d, then ##EQU2## and the critical radius of curvature becomes: ##EQU3##
The electrical resistance R of a conductor of a material with specific electrical resistance .rho., length 1, and cross-sectional area A, may be expressed as: ##EQU4##
The formula (2) implies that the critical radius of curvature is proportional to the modulus of elasticity, and inversely proportional to the allowable tension stress, and proportional to the diameter d for wires or fibers of circular cross-section. One alloy frequently used in conjunction with known cables is the platinum-iridium alloy, which consists of 10% by weight iridium and the rest platinum. This alloy has a comparatively high modulus of elasticity, specifically about 150 GPa, and a comparatively low allowable tension stress, specifically 0.3 GPa. If the wires are assumed to have a circular cross-section of a diameter of 0.1 mm, then, by inserting the above values into formula (2) there results a critical radius of curvature of 25 mm. However, the curvatures actually occurring when implanting a cable into the body, will be considerably smaller and thus below critical. This can be partially compensated by high-strength cobalt-alloys also used for making cables. These alloys have higher allowable tension stresses, to be sure, but even higher moduli of elasticity than the previously mentioned platinum-iridium alloy. Furthermore, it is difficult to form them to thin wires.
It is known in the field of superconductivity to inbed into a copper matrix a relatively large number of relatively thick wires of a titanium-niobium alloy, and to stretch the resulting composite conductor to thus make it thinner. There results a composite conductor containing titanium-niobium wires imbedded into a copper matrix and having diameters between 12 and 13 micrometers. Such a composite conductor cannot be used, however, as implant into a human or animal body. It is unsuited for such a purpose, because its outer diameter is of the order of magnitude of 1 mm, making the conductor very rigid, and also because copper may turn out to be toxic inside the body, as previously mentioned.